The present invention relates in general to waveform synthesizers for producing test signals, and more particularly to apparatus for generating test signals providing a reference standard for test and inspection of compatible quadrature amplitude modulation stereo transmission and reception equipment and similar equipment having complex modulation schemes.
In recent years, compatible quadrature amplitude modulation systems of stereo broadcasting, as marketed by Motorola, Inc. under their trademark "C-Quam", have begun to come into substantial commercial use. The "C-Quam" or compatible quadrature amplitude modulation system is a system of stereo broadcasting in which separate left and right stereo program information can be transmitted over ordinary AM broadcast stations and received in AM receivers equipped with a suitable decoder circuit. Also, ordinary receivers not so equipped with the decoder circuit can receive the program monaurally without serious distortion, and thus the system provides compatibility with ordinary AM receivers as well as the AM receivers equipped with the special decoder circuits. The "C-Quam" system is described in considerable detail in a number of Motorola, Inc. U.S. patents, including particularly U.S. Pat. Nos. 4,2l8,586 granted Aug. 19, 1980 and 4,338,491 granted July 6, 1982. Additional Motorola, Inc. patents describing the "C-Quam" system or components thereof include U.S. Pat. Nos. 4,159,396 and 4,159,398 granted June 26, 1979, 4,192,968 granted Mar. 11, 1980, 4,170,716 granted Oct. 9, 1979, 4,164,623 granted Aug. 14, 1979, 4,169,968 granted Oct. 2, 1979, 4,172,966 granted Oct. 30, 1979 and 4,371,747 granted Feb. 1, 1983.
The circuitry required to encode and decode the stereo program material is rather complex and requires critical tuning of many components. Both the encoder and the decoder must be so tuned. Ordinarily, under present practice they are tuned one against the other. It is not too difficult to obtain satisfactory encoding and decoding for the pair, but to my knowledge no direct and independent means have been developed to verify that the encoder generates the defined system signal. Presently, indirect means are used such as spectral analysis of the encoded signal to determine if the encoded signals are proper.
The equation for the compatible quadrature system or "C-Quam" system signal for the Motorola, Inc. system has been determined, the signal equation for such system being as follows: ##EQU1## where: E.sub.c =Complete "C-Quam" signal voltage for any time t
A.sub.c =Unmodulated carrier amplitude PA0 L(t)=Left channel program voltage at any time t PA0 R(t)=Right channel program voltage at any time t PA0 .omega..sub.c =2.pi.f.sub.c, rad/sec PA0 f.sub.c =Carrier frequency Hz PA0 t=Time, sec. PA0 M.sub.s =Modulation index for L+R program PA0 M.sub.d =Modulation index for L-R program PA0 50.pi.t=25 Hz pilot tone
The E.sub.c equation (1) shows that the carrier is amplitude modulated with the sum of the left and right program sources (L(t)+R(t)) and is phase modulated by the difference between the left and right program sources (L(t)-R(t)). Additionally, a pilot tone, which is the 50 .pi. t factor in the equation and in this embodiment is a 25 Hz pilot tone, of low level, phase modulates the carrier. The phase angle modulation is modified by the inverse tangent function (tan.sup.-1) to accomplish compatibility.
Observation of the equation (1) indicates that the generated signal is of a complex wave form. Present test signal generators utilizing analog elements experience troublesome stability problems. Thus, if such analog signal generators were used to generate a signal wave form in accordance with the "C-Quam" system signal equation, the generated test signal would drift and be distorted to such an extent that its use to test either an encoder or decoder would be impractical.